The genomic basis of adaptation in threespine stickleback fish

Evolutionary biology consists in the study of the evolutionary processes responsible of the diversification and adaptation of life forms over time. When adapting to a new environment (or changes in their local environment), populations have to adapt through natural selection. Until recently, the study of adaptation was focusing on fathoming the consequences of natural selection at the phenotypic level and how phenotypic evolution is linked to genetic changes. The development of new genetic and genomic tools in the last 20 years, like high-throughput sequencing technologies, now allows the construction of reference genomes in a variety of non-model organisms and the investigation of the genomic basis of adaptation. In my thesis, I investigated the genomic basis of adaptation by exploring the consequences of natural selection at the molecular level using the threespine stickleback fish (Gasterosteus acualeatus) as a model. In more detail, my work focused on three main topics: the genomic basis of parallel adaptation to acidic versus basic lochs of North Uist (Outer Hebrides, Scotland); the maintenance of standing genetic variation in Atlantic stickleback fish, and the characterization of reproductive isolation at the genomic level between parapatric stickleback populations of the Misty watershed (Vancouver Island, British Columbia, Canada)

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Classification of Lifshitz invariant in multiband superconductors: an application to Leggett modes in the linear response regime in Kagome lattice models

Multiband superconductors are sources of rich physics arising from multiple order parameters, which show unique collective dynamics including Leggett mode as relative phase oscillations. Previously, it has been pointed out that the Leggett mode can be optically excited in the linear response regime, as demonstrated in a one-dimensional model for multiband superconductors[T. Kamatani, et al., Phys. Rev. B 105, 094520 (2022)]. Here we identify the linear coupling term in the Ginzburg-Landau free energy to be the so-called Lifshitz invariant, which takes a form of $\boldsymbol{d}\cdot\left(\Psi^{*}_{i}\nabla\Psi_{j} - \Psi_{j}\nabla\Psi^{*}_{i}\right)$, where $\boldsymbol{d}$ is a constant vector and $\Psi_{i}$ and $\Psi_{j}$ $(i\neq j)$ represent superconducting order parameters. We have classified all pairs of irreducible representations of order parameters in the crystallographic point groups that allow for the existence of the Lifshitz invariant. We emphasize that the Lifshitz invariant can appear even in systems with inversion symmetry. The results are applied to a model of $s$-wave superconductors on a Kagome lattice with various bond orders, for which in some cases we confirm that the Leggett mode appears as a resonance peak in a linear optical conductivity spectrum based on microscopic calculations. We discuss a possible experimental observation of the Leggett mode by a linear optical response in multiband superconductors.Comment: 27 pages, 6 figure